SIMES: A Simulator forHybrid Electrical Energy Storage Systems

Siyu Yue, Di Zhu, Yanzhi Wang, Younghyun Kim,Naehyuck Chang, and Massoud Pedram

Background• Hybrid electrical energy storage (HEES) systemo Consists of several heterogeneous energy storage elements o Exploits the strengths (such as high cycle efficiency, high power density,

long cycle life, low cost, etc.) of each type of storage element and hide their weaknesses.

Control

DC/DC Converter

DC/AC Inverter

AC/DC Rectifier

DC/DC Converter

DC/DC Converter

DC/DC Converter

EES Bank #1 EES Bank #2

EES Bank #4

EES Bank #3AC Load

AC Source

CTI

Main Controller

Control

Status Status

• Some popular energy storage elements: Li-ion battery, lead-acid battery, Ni-Cd battery, supercapacitor, etc.

Background (cont’d)Applications of HEES systems:

Home application

Electric vehicle

Mobile device

Abstraction of HEES SystemsTop-level: We abstract a HEES system as a graph:

Source/load/bank/CTI => NodesConverters => Edges

Connection between a component (node) and a converter (edge) is abstracted as a port, which is simply a V-I curve.

Control

DC/DC Converter

DC/AC Inverter

AC/DC Rectifier

DC/DC Converter

DC/DC Converter

DC/DC Converter

EES Bank #1 EES Bank #2

EES Bank #4

EES Bank #3AC Load

AC Source

CTI

Main Controller

Control

Status Status

ConverterCport Vout

Iout

CportVin

IinIout=fc1(Vout,Vin,Iin,t)

V=fp(I,t)V=fp(I,t)

Modeling of HEES System ComponentsModeling of the energy storage banks:1. Open-circuit voltage as a function of state-of-charge2. Internal resistance as a function of state-of-charge3. Rate capacity effect4. Self-discharge effect5. State-of-health degradation (aging effect)

00.5110

11

12

13

14(1) OCV vs. SoC

SoC

OC

V(V

)

00.510

2

4

6

8(2) Internal Resistance vs. SoC

SoC

R(o

hm

)

ChargingDischarging

Fig 1. OCV and IR vs SoC curve of a lead-acid battery

0 1 2 3 4 520

40

60

80

100Discharge Efficiency vs. Discharge Current

Discharge Current(C)

Dis

cha

rge

Eff

icie

ncy

(%)

Measured ResultsCurve Fitting

Fig 2. Illustration of rate capacity effect

Modeling of HEES System Components Modeling of converters:

Reflects input/output voltage and current relations

Fig 3. Power conversion efficiency of a converter.

020

40

020

40

0.6

0.8

1

Vin (V)

(a) Efficiency at Iout

=0.6A

Vout

(V)

Effi

cien

cy

020

40

020

40

0.6

0.8

1

Vin (V)

(b) Efficiency at Iout

=1.6A

Vout

(V)E

ffici

ency

SIMES OverviewSIMES consists of three main modules:1. ParserParser parses the input XML file and

constructs the HEES system.2. SimulatorSimulator simulates the operation of the

HEES system.3. VisualizerVisualizer can help user create the input

XML file and visualize the output results.

Package Core

User

Visualizer(UI)

QT Libraries

DC/DC Converter

DC/AC Inverter

AC/DC Rectifier

DC/DC Converter

DC/DC Converter

DC/DC Converter

EES Bank #1 EES Bank #2

EES Bank #4

EES Bank #3AC Load

AC Source

CTI

Charge ManagerStatus Status

Control

ParserXML Parser

Simulator

Check Integrity

Adaptive Time-Step Simulation

Generate Output

CSourceBase CComponent

CBankBase

CPort

CLoadBase CConverterBase

CCTI

CController

CCTIPort

CSensor

Class Representation

Visualization of Results

SIMES

.xml Input

Output File

Implementation of SIMESSimulator runs adaptive time-step simulation.

At each time slot:•If it is a decision epoch, the main

controller sets the input or output current of each converter.

•Simulator calculates the current and voltage of each port.

•The maximum time-step which is acceptable for every component with regard to precision requirement is determined.

•Simulator simulates the operation of each component.

Check If Properly Set

Get Next Command

Cmd=Set/Get Property

Cmd=FinishCmd=Run Simulation till Time T

Current Time <T?

Set Load Current

MainController->Decision

Is Decision Epoch?

Converter->Solve

CTI->FindRegulatorCurrent

Determine min(TimeStep)

Yes

Component->Simulate

No

Yes

Simulator workflow

Sensor Output

Output File

Write To

Exit

No

Implementation of SIMESVisualizer is implemented using QT5.0 libraries.

Validation of SIMESWe compare SIMES’s simulation result with a HEES system prototype.

0 500 1000 150018

20

22

24

26(a) CTI Voltage

Vo

ltag

e (

V)

Time (s)0 500 1000 1500

22

23

24

25(b) Li-ion battery bank

CC

V (

V)

Time (s)0 500 1000 1500

-2

0

2

Cu

rre

nt (

A)

0 500 1000 150018

20

22

24

26(c) Lead-acid battery bank

CC

V (

V)

Time (s)0 500 1000 1500

-2

0

2

Cu

rre

nt (

A)

0 500 1000 15000

8

16

24

32

40(d) Supercapacitor bank

CC

V (

V)

Time (s)

0 500 1000 1500-5

0

5

Cu

rre

nt (

A)

Voltage(Measurement)

Voltage(Simulation)

Current

Fig 4. Simulation results vs. hardware measurement

Hardware Measurement Time

Simulator Runtime

30min 1.1s

Use Case Demonstration (1)•A single-family house equipped with a HEES system consisting of a Li-ion

battery and lead-acid battery.•This system draws energy from the grid when the electricity price is low and

supply energy to the grid when the price is high.•SIMES simulates the operation of such a system for one day and determine its

efficiency.

0 4 8 12 16 20 240

0.5

1

Sta

te o

f Cha

rge

Time (h)0 4 8 12 16 20 24

-5

0

5

Cur

rent

(A

)

Time (h)0 4 8 12 16 20 24

15

20

25

30

Vol

tage

(V

)

Time (h)

Li-ion

Lead-acid

Fig 5. SoC, current and voltage of both banks

Total energy drawn from the grid is 1575 Wh and supplied to the grid is 1266 Wh.Overall efficiency: 80.4%

Use Case Demonstration (2)•A mobile device with a HEES system consisting of a Li-ion battery and

supercapacitor.•An expert-based online management algorithm is implemented in the mobile

device which determines how to charge/discharge the supercapacitor to minimize the power loss in the system.

•SIMES simulates its operation and calculate the power consumption and power loss in the system. The baseline is using a Li-ion battery only as the power supply.

0 0.2 0.4 0.6

Power (W)

(a) Overall Power Consumption

Li-ion

Hybrid

Delivered power

0 0.02 0.04 0.06 0.08 0.1

Power (W)

(b) Power Loss

Li-ion

Hybrid

Power loss

Fig 6. Power consumption and power loss of the mobile device

Power consumption is reduced by 4% and power loss is reduced by 28%.

Q&A

Thank You!